For everybody who is interested on internals of Entity Framework (ER) in relation to referential integrity, I would recommend this ADO.NET team article written by elisaj: http://blogs.msdn.com/adonet/pages/how-referential-integrity-works.aspx.
Another interesting article, which describes ER performance can be read here: http://blogs.msdn.com/adonet/archive/2008/02/04/exploring-the-performance-of-the-ado-net-entity-framework-part-1.aspx and here: http://blogs.msdn.com/adonet/archive/2008/02/11/exploring-the-performance-of-the-ado-net-entity-framework-part-2.aspx.